Fast string of horses (6)
I believe the answer is:
stable
I believe this is a triple definition!
'fast' is the first definition.
(fast can mean secure)
'string' is the second definition.
(I've seen this before)
'of' acts as a link.
'horses' is the third definition.
(I've seen this before)
(Other definitions for stable that I've seen before include "Fixed and unchanging" , "'Steady, not given to sudden change (6)'" , "Unchanging (condition); some racehorses" , "Firm - building for horses" , "Not likely to change" .)